Transformer-coupled voltage converters are very common and are frequently used to generate a desired voltage at an output stage, where the output stage is electrically isolated from the input stage. The output voltage is typically different from the input voltage and may be a regulated voltage, a non-regulated voltage, a stepped up voltage, a stepped down voltage, or an inverted voltage. A transformer-coupled converter is particularly useful if a common ground is not desired.
Transformers typically have some leakage inductance due to imperfect coupling of the windings via the core, resulting in the creation of leakage flux. The leakage flux alternately stores and discharges magnetic energy with each electrical cycle and thus effectively acts as an inductor in series in each of the primary and secondary circuits. The leakage flux is typically wasted, reducing the efficiency of the converter.
FIG. 1A illustrates a prior art push-pull regulator 10, which is a type of forward converter. The unregulated DC input voltage (Vin) is filtered by a capacitor Cin and is applied to the center tap of the primary winding of a transformer T. Bipolar transistors Q1 and Q2 alternately conduct to alternately energize each side of the primary winding. The currents through the primary winding are magnetically coupled to the secondary winding to generate currents through the secondary windings. The currents and voltages produced at the output of the transformer depend on the winding turns ratio, the switching frequency, the duty cycle, the sizes of the components, and other factors. The pulses generated by the secondary windings are rectified by diodes D and filtered by inductor L and capacitor Cout to provide a DC regulated voltage Vo at the output of the regulator 10.
A control circuit 14 receives the output voltage, or a scaled version of the output voltage, and compares this feedback voltage to a reference voltage. The control circuit then adjusts the frequency or duty cycle of switching to keep the feedback voltage matched to the reference voltage.
If the input voltage is a battery or other variable voltage, then feedback is needed to control the output voltage. If regulation is not required, feedback is not necessary.
The maximum voltage across each of the transistors Q1 and Q2 is ideally only double the input voltage Vin. However, due to leakage inductances in the windings, high voltage spikes may appear across the transistor Q1 or Q2 after the transistor turns off.
FIG. 1B illustrates the voltages across either transistor Q1 or Q2. When a switch Q or Q2 is on, the voltage across that switch is essentially zero volts. When neither switch is on, the voltage across both switches is Vin. When one switch is on and the other is off, the voltage across the off switch is 2Vin due to the transformer voltage being “reflected” from the active primary winding to the inactive primary winding.
Voltage spikes occur during switching due to leakage inductance, where energy is not perfectly transferred from the primary winding to the secondary winding. Such spikes may be hundreds of volts. To prevent such spikes, snubber circuits are sometimes added to dissipate that energy. In FIG. 1A, the snubber circuits are the RC filters (Rs1/Cs1 and Rs2/Cs2). Such energy is wasted in the snubber circuits.
It would be desirable to recover the energy normally wasted by the snubber circuits.